Part II: Evidence of the link between animal agriculture and antimicrobial resistance

Yesterday, I wrote about the basics of animal agriculture and antimicrobials. Today, I’ll dive deeper into the issues.

Part II: Evidence of the link between animal agriculture and antimicrobial resistance

What is antimicrobial resistance and why should I care about it? Antimicrobial resistance occurs when microbes have developed the ability to evade antimicrobials, survive antimicrobial treatment, multiply, and infect others. Microbes are able to survive partly because antimicrobial treatment may kill off the sensitive microbes and leave the more adapted ones to adapt to the antimicrobial and multiply.

Microbes can become resistant to multiple drugs. This makes the infection difficult or impossible to treat. By its very nature, an infection will spread to others, endangering more people with resistant infections.

The FDA has a pretty great video explaining the process of antimicrobial resistance.

Is there any evidence of association between antimicrobial use in animal agriculture and antimicrobial resistance in humans? Yes. Most of the evidence is based on studies of foodborne illness such as Salmonella and Campylobacter because the foodborne route is the most common way that resistant microbes are transferred from animals to humans.

Some resistant bacteria will themselves endanger human health. Others which cannot make humans ill will share their resistant genetic code with microbes that can make humans ill. These previously vulnerable, pathogenic microbes become resistant when they receive the resistant genes.

Using antimicrobials at sub-therapeutic levels to enhance growth means that all bacteria in an animal’s body is regularly exposed to low levels of antimicrobials. The most susceptible microbes will be killed or incapacitated, but the surviving ones will become increasingly resistant to the antimicrobial used.

How does using antimicrobials in animal agriculture contribute to human foodborne illness? The CDC report “Antibiotic resistance threats in the United States, 2013” outlines exactly how these two issues are related:

  1. Antimicrobial-resistant microbes may be formed through biological (e.g. selective pressure, mutations) or human (e.g. antimicrobial misuse, inadequate diagnostics) avenues.
  2. Antimicrobials used in animal agriculture kill off susceptible microorganisms while allowing resistant microbes to survive.
  3. Resistant microbes can be passed from animals to humans through fecal or other forms of contamination of food.
  4. When humans eat contaminated food, they develop infections (e.g. coli) that cannot be treated with antimicrobials. For generally healthy people, this may not be problematic, as their immune system will fight the infection itself. However, some people will need a boost from antimicrobials—antimicrobials that are now useless.

Beacause of this strong connection between animal antimicrobial use and human illness, CDC recommends that antimicrobials are used only to treat infections rather than to enhance growth. The CDC calls this antibiotic stewardship. 

What are some other ways animal agriculture-induced antimicrobial resistance affects human health?

  • Infections that would not have otherwise occurred
  • Treatment failures
  • Increased severity of infections (Source.)

Is animal agriculture the only cause of antimicrobial resistance? Definitely not. The other major contributor to antimicrobial resistance is improper human medical use. For example, when doctors prescribe antibiotics for a viral infection, the antibiotic will not treat the viral infection. However, the antibiotic may kill off a few bacteria from a minor bacterial infection, leaving only the remaining bacteria resistant to the drug.


Come back tomorrow for Part III: The Stuff That Will Keep You Up At Night